1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer used in optical communications, and more particular to a multichannel array waveguide diffraction grating optical multiplexer/demultiplexer and a method of connecting an array waveguide and output waveguide.
2. Description of the Related Art
In the field of optical communications, a method (wavelength dividing multiplexing method) has been put into practical use to increase the capacity of transmission by sending a large capacity signal by using a high-frequency carrier wave.
In this method, an optical multiplexer/demultiplexer which multiplexes and demultiplexes signals with different wavelengths takes an important role.
Particularly, an array waveguide diffraction grating optical multiplexer/demultiplexer using an array waveguide diffraction grating (AWG) is useful for multiplying the number of channels. Regardless of the number of channels, a desired number of channels can be created in the same process and same number of steps, and in principle, loss and deterioration in characteristics are small.
When connecting an array waveguide diffraction grating (AWG) and an optical fiber as an output waveguide, a minimum coupling loss is demanded.
For example, Jpn Pat. Appln. KOKAI Publication No. 11-271557 proposes an example of a coupler comprising first waveguide line array has an M-channel and second waveguide line has an N-channel waveguide array, which has a pair of end portions, one end portion connected to an arc-shaped planer waveguide line having a center at the middle of the other end portion, wherein the N-channel second waveguide line array is arranged radially from the center point of the arc placed near the center of the end portion of the first waveguide line array.
However, it is generally known that if multichannel (N-channel) waveguides (waveguide lines) are arranged on the circumference of a Rowland circle drawn on the surface of curved diffraction grating to contact at the middle point, the asymmetry is different in the transmission characteristics of the waveguide (waveguide line) placed near the center of the N-channel waveguides (waveguide lines) and the waveguides (waveguide lines) placed at the terminal ends (both ends).
If the asymmetry of transmission characteristics is different in a pass bandwidth, PDL is locally degraded (the worst value of PDL increases). Further, in an array waveguide with flat top transmission characteristics, ripple in the pass bandwidth increases.
In Patent Document 1, there is no suggestion about a method of solving the above problems.